1. Field of the Invention
The invention relates to a process for producing polycrystalline silicon.
2. Description of the Related Art
High-purity polycrystalline silicon (polysilicon) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone-melting (FZ) processes, and for production of mono- or multicrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics.
Polysilicon is typically produced by means of the Siemens process. This involves introducing a reaction gas comprising one or more silicon-containing components and optionally hydrogen into a reactor comprising support bodies heated by direct passage of current, silicon being deposited in solid form on the support bodies. Silicon-containing components used are preferably silane (SiH4), monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) or mixtures of the substances mentioned.
The Siemens process is typically conducted in a deposition reactor (also called “Siemens reactor”). In the most commonly used embodiment, the reactor comprises a metallic base plate and a coolable bell jar placed onto the base plate so as to form a reaction space within the bell jar. The base plate is provided with one or more gas inlet orifices and one or more offgas orifices for the departing reaction gases, and with holders which help to hold the support bodies in the reaction space and supply them with electrical current. Each support body usually consists of two thin upright filament rods and a horizontal bridge which connects the generally adjacent rods at their free ends. The filament rods are inserted vertically into electrodes present at the reactor base, through which they are connected to the power supply.
Deposition of polysilicon is accomplished by opening a shutoff valve for the reaction gas flowing into the reactor, a mixture of hydrogen and one of the aforementioned silicon-containing components, and a shutoff valve for the offgas flowing out of the reactor. The reaction gas flows through the feed orifice in the base plate into the closed deposition reactor. The silicon is deposited therein on the thin rods heated by direct passage of current. The hot offgas formed in the reactor leaves the reactor through an orifice in the base plate and can then be subjected to a processing operation, for example a condensation, or be sent to a scrubber.
High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the diameter thereof increases with time. Once the desired diameter has been attained, the process is stopped. The polycrystalline silicon rod pairs formed are then typically cooled to room temperature.
U-shaped polycrystalline silicon rods are obtained, which may be several meters in height and may weigh several thousands of kilograms.
After cooling to room temperature, the silicon rods can be removed from the reactor. Since the Siemens process is conducted batchwise, the reactors are shut down for this purpose, i.e. the deposition time is interrupted. At this time, the shutoff valves for reaction gas are closed and the power supply is interrupted. The deposited polycrystalline silicon can be removed from the opened reactors. To open the reactors, an upper reactor section is removed, typically raised upward.
From an economic point of view, it is advantageous to make the disassembly and setup times as short as possible, in order to keep the time before the subsequent deposition batch as short as possible.
It is necessary to deinstall the polycrystalline rod pairs from the reactor in one piece (two rods and one bridge). Full harvesters which can remove all rod pairs at the same time are also known, as will be explained hereinafter.
In the event of imprecise removal, one or more rods or rod pairs may fall over and break up in the process or on contact, for example, with the shop floor, with the base plate of the reactor, or with other rods or rod pairs. If several rods are present in the reactor, there is a very high risk that all the rods in this batch will then fracture and be unusable for further processing because of the fracturing and possible contaminations.
After rod deinstallation, it is customary to clean the bell jar and base plate of the reactor and to provide the reactor with new electrodes and thin rods for the next deposition batch. After the bell jar has been closed, the process for deposition of the next batch of polysilicon is conducted again as described above.
The opening of the reactor and the rod deinstallation should be effected in such a way as to avoid surface contamination of the rods and to minimize breakage of the rods.
JP7029045B describes a disassembly rocker which approaches the side of the reactor and lifts the rod pairs out. This process requires the complete disassembly of the reactor apart from the base plate. The risk here is that the rod pairs are removed only at the electrodes by the prongs. In the course of this, tilting of the rod pair in the wrong direction cannot be ruled out. A rod pair in an uncontrolled fall can mean not just considerable endangerment of personnel by splinters but also the loss of an entire batch if this rod pair takes the rod pairs still remaining upright in the reactor down as well.
US 20120237678 A1 discloses an apparatus for deinstalling polycrystalline silicon rods, comprising a body having outer walls and having such dimensions that the rods are surrounded by the outer walls, each outer wall including a door to allow access to at least one of the rods. In a preferred embodiment, the inner walls are lined with a polymer to prevent contamination of the polycrystalline silicon rods.
US 20100043972 A1 discloses a further apparatus for deinstalling polycrystalline silicon rods, comprising a wall comprising an inner wall, an outer wall and multiple wall connectors connecting said inner wall with said outer wall, a cavity formed between said inner wall and said outer wall, an access window formed in said outer wall, a base plate, and a plurality of contacts disposed on said base plate, wherein said inner wall and said outer wall are cylindrical and concentric, said cavity is adapted to receive a plurality of silicon rods resting on said contacts, and said access window is adapted to provide access to the silicon rods. The rods can be removed via the access window.
However, the problem is that apparatuses of this kind have a high space requirement between the rod pairs. A high space requirement, however, is disadvantageous for achievement of high economic viability (enablement of high final diameters).
An additional disadvantage of the above-described apparatuses is that, in the case of rods standing askew or in the case of a partially collapsed batch, which is not a rare occurrence, it is impossible to use them.
This also applies to the method claimed in DE 10 2009 027 830 B3 for removing polycrystalline silicon rods from a reactor, which comprises running a rigid and automated guide over the opened reactor with a computer-assisted recognition method on the basis of calibration points and taking hold of the rod pairs by means of a mechanical or pneumatic clamping apparatus and then laying them down in a transport apparatus.
JP 63296840 A discloses an apparatus for deinstalling silicon rods from a deposition reactor, in which an individual rod pair is fixed with the aid of clamps and lifted out of the side of the reactor. JP 2002210355 A likewise discloses an apparatus for deinstallation of silicon rods, comprising an arm which is movable in three dimensions and has a clamp apparatus mounted at the end thereof, which can be used to lift the silicon rods out of the reactor.
A disadvantage of these two apparatuses is the fact that the rods can be removed from the completely opened reactor only from the outside inward. A selected deinstallation of a particular silicon rod, for example from an inner rod circle, which is sometimes desirable, is impossible with the apparatus described.
A further disadvantage is that this system has to be manually actuated, and the manual coordination of this multiaxial system is very difficult. As a result, no time advantage is achieved over conventional removal apparatuses. A further disadvantage is the strong flexural forces that act on this construction as the rods are being lifted out. In the deinstallation process, a certain pulling force has to be applied, which causes the arm to give when the rods are detached. This giving of the arm on deinstallation can lead to adjacent rod pairs being touched and knocked over by the deinstallation tool. However, the main disadvantage of this system is that, for deinstallation, all the rods have to be at least partly in a clear space to be grasped by the gripper arm. If one or more rods falls over, this will inevitably lead to contamination of the silicon and may even cause serious injury to personnel.
US 20120175613 A1 discloses a method for producing a polycrystalline silicon piece, consisting of a CVD process for production of a polycrystalline silicon rod by deposition of silicon on a filament wire, one end of which is attached to a first electrode and the other end of which to a second electrode, a process for removing the polycrystalline silicon rod from the reactor and a process for comminuting the silicon rod into silicon pieces, which comprises removing at least 70 mm from the electrode end of the polycrystalline silicon rod prior to the comminution process (base shortening process). In a preferred embodiment, the surface of the polycrystalline silicon rod is covered with a bag-like polyethylene component before being removed from the reactor. The removal itself can be effected by means of a crane or the like.
The procedure disclosed in the prior art and apparatuses used have disadvantages in that the reactor section always has to be lifted away in the course of deinstallation, such that the rod pairs at least temporarily stand exposed on the base plate with nothing to prevent them from falling over. This constitutes a considerable safety risk, since rods or parts of rods falling over may cause considerable injury to personnel working alongside or in the deposition plant. Should the deinstallation proceed through an additional upper flange in the reactor, there is the disadvantage that the bell jar of the corresponding deposition plant cannot be cleaned until the deinstallation of the rods is complete. In this procedure, the time required for the batch changeover (deposition-free time) is much higher than if the bell jar could be cleaned at the same time and if it were not necessary to wait for the completion of cleaning after the deinstallation.